Non-volatile memory devices require a number of different operative voltages for performing read/write operations on their memory cells (for example, relatively high voltages are needed to program and/or erase the memory device). Typically, the memory device includes a power management unit, which generates the different operative voltages from a single power supply voltage; in this way, it is possible to avoid the need of providing multiple external power supply voltages.
For each operative voltage, the power management unit generally includes a charge pump that boosts the power supply voltage to the desired value. The charge pump is coupled with a corresponding regulator, which maintains the operative voltage substantially constant under changing load conditions (in order to avoid any damage to the memory cells).
Several solutions are known in the art for implementing the regulator of each charge pump. For example, a circuit scheme based on a resistive divider is routinely used when a high accuracy of the operative voltage is required (such as for the programming operation). In this case, a measuring voltage is derived from the operative voltage, through a resistor that is biased by a current obtained from the reference voltage (through a further resistor); the measuring voltage is compared with a reference voltage (for example, provided by a band-gap circuit), and the charge pump is controlled accordingly. Therefore, the regulation only depends on the reference voltage (being accurate in its nature) and the ratio between two resistances (which ratio can be controlled with high accuracy when the memory device is realized as an integrated circuit).
Different solutions for the regulator are based on diode dividers or current measures. Those solutions find application when a lower accuracy of the operative voltage is acceptable (for example, for the reading operation).
A drawback of the power management units known in the art is that each charge pump is regulated independently.
Therefore, it is not possible to ensure a common level of quality in the regulation.
Moreover, each regulator must be trimmed individually to provide the desired accuracy.
Such a trimming operation is very time-consuming, and then reduces the throughput of a corresponding manufacturing process of the memory devices.
In any case, the accuracy of the reference voltage must be very high to ensure that any error in the operative voltages remains within an acceptable range. For example, in most applications the error of the reference voltage cannot exceed ±40 mV. The above-described requirement is quite stringent, and adversely affects the yield of the manufacturing process.